1. Field of the Invention
The present invention relates to a thin film transistor, a display device using the thin film transistor, and a production method of the thin film transistor. More particularly, the present invention relates to a double-gate thin film transistor, a display device using the double-gate thin film transistor, and a production method for the double-gate thin film transistor. Note that, in this specification, the term “double-gate” refers to a structure in which gate electrodes are disposed on an upper side (one surface side) and a lower side (another surface side) of a semiconductor layer through an intermediation of gate insulating films.
2. Description of the Related Art
A thin film transistor is a kind of a field-effect transistor and is mainly applied to an active matrix liquid crystal display or an active matrix organic electroluminescence (EL) display. At present, the mainstream is a polycrystalline silicon thin film transistor in which polycrystalline silicon is used as a semiconductor layer and an amorphous silicon thin film transistor in which amorphous silicon is used as a semiconductor layer. With respect to a polycrystalline silicon thin film transistor in which a mobility of electrons is several hundred times as large as that in an amorphous silicon thin film transistor, not only a technology for a type formed at high temperatures but also a technology for a type formed at low temperatures has been completed, and an increase in size of a panel which has been difficult is now expected to be realized. On the other hand, an amorphous silicon thin film transistor is suitable to achieve a large area of a panel and reduce cost in the production process thereof, and has therefore become a mainstream of the thin film transistor and is utilized in various fields.
In recent years, as a new kind of a thin film transistor, a thin film transistor in which a transparent oxide polycrystalline thin film mainly formed of ZnO is used as a semiconductor layer has been actively developed (Japanese Patent Application Laid-Open No. 2002-76356). The above-mentioned thin film may be formed at low temperatures and is transparent to visible light, and thus, a flexible and transparent thin film transistor may be formed on a substrate such as a plastic plate or a film. Further, K. Nomura et al., Nature, Vol. 432 (2004-11), (English version), pp. 488-492 discloses a technology in which a transparent amorphous oxide semiconductor formed of indium, gallium, zinc, and oxygen is used as a semiconductor layer of a thin film transistor. It also describes that a flexible and transparent thin film transistor having a large mobility may be formed on a substrate such as a polyethylene terephthalate (PET) film at a room temperature.
As a common issue for the above-mentioned thin film transistors, there is deterioration of electrical characteristics when the semiconductor layer is exposed to light. Specifically, carriers are generated in the semiconductor layer by light irradiation, and therefore, absolute values of an ON voltage and an OFF current in drain current (Ids)-gate voltage (Vgs) characteristics (transfer characteristics) are increased. In C. van Berkel et al., Journal of Applied Physics, 60 (1986) p. 1521, it is described that an OFF current increases when an amorphous silicon thin film transistor is irradiated with light having a wavelength of 600 nm. Further, in D. P. Gosain et al., 7-2, AM-FPD2008, it is described that the ON voltage is significantly shifted to a negative side when a transparent amorphous oxide semiconductor thin film transistor is irradiated with light beams having various wavelengths. For example, in an active matrix liquid crystal display or an active matrix organic EL display, malfunctions such as deterioration in contrast occur because the OFF current in the transfer characteristics increases. Further, occurrence of the shift in ON voltage makes conspicuous such malfunctions that a desired threshold value which is necessary for operating a circuit may not be obtained or an operating point of the circuit is shifted. For example, a thin film transistor for driving in a pixel circuit of an active matrix organic EL display controls emission intensity of an organic EL device by a current flowing between a drain and a source thereof. Therefore, if the ON voltage is shifted, a malfunction occurs in which desired emission intensity may not be obtained.
In order to solve the above-mentioned problems, the following method is generally used. That is, in order to prevent light irradiation onto a semiconductor part of the thin film transistor, an upper light shielding film and a lower light shielding film, which are formed of a material which shields light, are formed on an upper portion and a lower portion of the thin film transistor, respectively. Further, Japanese Patent Application Laid-Open No. 2002-196362 and Japanese Patent Application Laid-Open No. 2004-170656 disclose a technology in which, in addition to the upper and lower light-shielding layers, a side-wall light-shielding layer for shielding light entering from side surfaces of the semiconductor is formed to be connected to the upper and lower light-shielding layers, to thereby effectively shield light irradiating the polycrystalline silicon semiconductor layer. Further, US 2007-090365 A1 discloses a technology related to a light-shielding layer in a thin film transistor using an oxide semiconductor as the semiconductor layer. According to US 2007-090365 A1, even if a transparent amorphous oxide semiconductor, which is transparent to visible light, is used, when light in a particular visible light region enters, a change in electrical conductivity caused by light occurs. However, by forming light-shielding layers separately from a source electrode, a drain electrode, a gate electrode, and an active layer, the thin film transistor may be more stably operated.
According to the inventions disclosed in Japanese Patent Application Laid-Open No. 2002-196362, Japanese Patent Application Laid-Open No. 2004-170656, and US 2007-090365 A1, all the upper, lower, and side-wall light-shielding layers are formed separately from layers constituting the thin film transistor, that is, the gate electrode, the gate insulating film, the active layer, the source electrode, and the drain electrode. Therefore, steps of forming the light-shielding layers are necessary separately from the steps of forming the thin film transistor, which leads to an increase in the number of production steps.